1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device having a superjunction structure.
2. Background Art
Vertical MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) suitable for power electronics application are conventionally known. The on-resistance of this MOSFET greatly depends on the electric resistance of the conduction layer (drift layer). The resistance can be decreased by increasing the dopant concentration in the drift layer. However, there is a limitation on the increase of dopant concentration for ensuring a desired breakdown voltage. That is, there is a tradeoff between the device breakdown voltage and the on-resistance. Improving this tradeoff is important for low power consumption devices.
As an example MOSFET improving the tradeoff, a MOSFET having a structure called the “superjunction structure” is known, where p-type pillar regions and n-type pillar regions are provided in parallel in the drift layer. In this structure, a non-doped layer is artificially produced by equalizing the amount of dopant contained in the p-type pillar region and the n-type pillar region. While maintaining high breakdown voltage, a current is allowed to flow through the n-type pillar region doped with high concentration. Thus a device with low on-resistance over the limit of the material is realized.
Furthermore, in a semiconductor device having a superjunction structure, the breakdown voltage and avalanche withstand capability depend not only on the structure of the device section, but also on the structure of the terminal section For example, JP 2003-115589A discloses a semiconductor device having a superjunction structure in the terminal section as well as in the device section. However, this configuration has a large decrease of breakdown voltage in the terminal section versus the fluctuation of the amount of dopant.
Moreover, if the terminal section has no superjunction structure, electrons and holes generated upon avalanche breakdown cause the electric field in the top and bottom portion of the terminal section to increase. This increases the breakdown current, which is likely to destroy the device. That is, the device has a low avalanche withstand capability.